1. Field of the Invention
This invention relates to friction stir welding and, more particularly, to an anvil used in friction stir welding processes that is either coated with or manufactured from a substance capable of resisting anvil deformation and preventing diffusion bonding or mechanical bonding between the anvil and the friction stir welding tool or work piece when friction stir welding high temperature materials or work pieces.
2. Background and Related Art
Friction welding has been used for years in the welding industry. For example, when the ends of two pipes are pressed together and simultaneously turned in opposite directions to one another heat is generated and causes the ends of the pipes to become plasticized and bonded. Rapidly stopping rotation of the pipes causes the two pipes to fuse together.
Friction stir welding, on the other hand, is a relatively new technique, and was first described in U.S. Pat. No. 5,460,317, issued Oct. 24, 1995. Friction stir welding involves pressing a non-consumable friction stir welding tool with a profiled probed end against two work pieces at the point where they come in contact with one another. Cyclical movement of the probed end of the tool generates heat as it presses against the two work pieces. The probed end of the tool enters or plunges into the two work pieces in a plasticized region created by the friction of the probed end. The probed end can then be slowly moved to a region where the two pieces abut each other, thereby enabling the area between the two pieces, which is beneath the probed end, to solidify together.
There are a number of advantages to friction stir welding. Among these include the fact that during friction stir welding the material heated is not exposed to combustion products. This reduces chemical changes in the work pieces due to the interface of the work pieces with the tool and its byproducts. Another advantage of friction stir welding is that the temperature of the work pieces, even in the heated region, tends not to be as high as the temperature resulting from conventional welding processes. This reduced temperature reduces oxidation of the work pieces due to ambient atmosphere, thereby reducing the need to provide an inert atmosphere at the weld location.
Friction stir welding has traditionally been limited to welding of low melting temperature materials such as aluminum alloy, copper alloys, lead and magnesium alloys because of the wear on the probed end. These materials are effectively joined using standard steel backing supports or anvils. However, recent advances in technology allow for the friction stir welding of harder, previously unweldable materials.
In order to join so-called ‘higher temperature materials’ such as steels, stainless steels, nickel alloy or titanium alloys, it is necessary to employ increased temperatures and forces. These increased temperatures and forces present new problems. For instance, such high temperature materials can lead to diffusion or mechanical bonding of the work pieces to the anvil. Moreover, friction stir welding of high temperature work pieces may cause unwanted and added deformation of the anvil.
The current trend in the art when joining high temperature materials is to avoid full penetration with the probed end so as to reduce the probability that diffusion or mechanical bonding of the work pieces to the anvil occurs. Unfortunately, if the probed end penetrates only a small amount into the joint, only a portion of the joint is heated and joined, and the weld is not as strong as when the weld is fully penetrated through the joint region. In addition, the material is not fully plasticized in the entire welded seam, causing a portion of the apparently welded region to be in the form of a lap weld, which is a region where the material from one of the work pieces overlaps onto, but does not fully bond with, the material of adjacent work pieces. The weakness of such a weld is not always obvious or at least visible upon a precursory inspection.
Recent trends in the art attempt to achieve full penetration welds. A number of inventions deal with adding a chamfer or a groove to the anvil or the work pieces. (See U.S. Pat. Nos. 5,611,479 and 5,769,306.) Additionally, a number of patents focus on using a feedback control system to control the depth of the pin, thereby controlling the level of penetration. (See U.S. Pat. Nos. 6,168,066 and 6,173,880.)
U.S. Pat. No. 6,168,066 incorporates a ceramic sensor plate for sensing magnetic fields and eddies to control the depth of the magnetic friction stir welding tool in softer non-magnetic materials.
Accordingly, it would be an improvement in the art to augment or even replace the current techniques with other techniques.